1. Field of the Invention
The present invention relates generally to a technology for wavelength conversion from a fundamental wavelength laser beam to a harmonic light beam and, more particularly, to a method and apparatus for generating a long-pulse harmonic pulse laser beam suited for laser processing such as welding etc.
2. Description of the Related Art
Recently, a laser is have been utilized in fields of manufacturing, especially welding, cutting and surface finishing. In fact, laser welding technology is increasingly becoming important because highly accurate and high speed processing can be achieved; a workpiece is affected by less thermal strain; and advanced automation can be enabled. Currently, a solid laser is used most frequently for laser welding and is a YAG laser which generates a light beam with a wavelength of about 1 μm. The YAG laser comprises a YAG (Y3Al5O12) crystal doped with rare earth active ions (such as Nd3+, Yb3+ and the like) and a fundamental wavelength of Nd:YAG laser is 1064 nm. The YAG laser enables continuous oscillation and giant pulse oscillation with Q switch to be achieved and can generate a long-pulse laser beam with a pulse width of more than 100 μs (typically, 2 to 3 ms).
By the way, in the laser welding, optical coupling between a welded material and a laser beam is important. Without good optical coupling, since reflectance becomes higher and absorption efficiency of laser energy becomes lower, it is difficult to obtain a good welding connection. In this regard, the YAG laser beam with a fundamental wavelength (e.g. 1064 μm) has poor optical associativity to copper, gold, aluminum and the like. To these metals, it is known that a second harmonic (532 nm) YAG laser has higher optical coupling. In a patent document 1, the applicant has disclosed a different wave length superimposed laser welding method which generates the fundamental wavelength (1064 nm) YAG pulse laser beam with a first YAG laser as well as generates the second harmonic (532 nm) Q switch YAG laser beam with a second YAG laser to coaxially superimpose and irradiate the fundamental wave YAG pulse laser beam and the second harmonic Q switch YAG laser beam on welded material. According to this different wave length superimposed laser welding method, for the welded material such as copper, gold, aluminum and the like, absorption efficiency of laser energy can be enhanced to obtain good welding connection (see, e.g., Japanese Patent Application Laid-Open Publication No. 2002-28795).
However, in the different wave length superimposed laser welding method described above, since two (2) YAG laser systems are required, it is problematic that a laser apparatus becomes larger and adjustment and maintenance become complicated.